The present invention relates to a process for the production of calcium carbonate, and in particular, to a process for the production of calcium carbonate particles of uniform size in the range of 0.3.+-.0.1 .mu.m, which are suitable as a pigment for coated papers.
Throughout the specification and appended claims, the term "size" of calcium carbonate particles refers to the length of one edge of the particles, as measured by electron microscopy analysis, and parts and percentages are all by weight unless otherwise specified.
The calcium carbonate used in industry may be categorized into two groups, i.e., heavy calcium carbonate and precipitated calcium carbonate. Heavy calcium carbonate is prepared by pulverizing limestone into fine particles and then classifying the resultant powders. On the other hand, precipitated calcium carbonate having an average particle size not greater than 5 .mu.m is prepared by reacting calcium and carbonate ions.
Precipitated calcium carbonate particles are manufactured by "liquid-liquid" process or by "liquid-gas" process. In the liquid-liquid process, a solution containing ions, such as sodium carbonate and ammonium carbonate, is reacted with a solution of a calcium compound, such as calcium chloride and calcium acetate. The liquid-gas process is further divided into a so called "forward reaction" process, in which a carbon dioxide-containing gas is introduced into an aqueous suspension of calcium hydroxide, and a so called "reverse reaction" process, in which a calcium hydroxide suspension is added into a carbon dioxide-saturated water. According to the liquid-gas process, so called colloidal calcium carbonate having a cube-like particulate configuration and an average particle size of about 0.04 .mu.m, or so called light calcium carbonate having a spindle-like particulate configuration and an average particle size greater than 1 .mu.m are obtained. These colloidal and light calcium carbonate particles are currently used as fillers in various industrial fields, such as rubbers, plastics, paints, inks, sealants, and papers.
In particular, fine-sized calcium carbonate having a uniform particle size in the range of 0.1 to 1.0 .mu.m, which shows reduced aggregational affinity and improved characteristics of dispersibility and fluidity and imparts high opacity and brightness to coated papers, is in strong demand. Colloidal calcium carbonate particles having an average size less than 0.1 .mu.m show strong affinity of aggregation, and may not provide high opacity to the end product. On the other hand, light calcium carbonate particles are not suitable as a pigment for paper coatings, since a slurry thereof shows a strong dilatant property, although a thixotropic property is required in the paper-coating process.
In a conventional process, it is suggested to heat and age a suspension containing calcium carbonate particles of 0.04 .mu.m in average size until the particles grow to an average size of about 0.08 .mu.m. However, calcium carbonate particles manufactured by this process have widely varying sizes, and the process itself is not economical.
U.S. Pat. No. 3,848,059 provides a process, in which spheroidal shaped reticulated precipitated calcium carbonate of 0.1 to 5 .mu.m is prepared by the double decomposition of two water-soluble salts such as calcium chloride and potassium carbonate dissolved in water droplets of two water-in-oil emulsions.
U.S. Pat. No. 4,018,877 (R. D. A. Wood) describes a carbonation process in which a complex-forming agent for calcium ions is added to an aqueous suspension of calcium hydroxide after the calcium carbonate primary nucleation stage, but before completion of the carbonation stage. Optionally, a long-chain fatty acid or long-chain fatty acid salt can be added, preferably after a final carbonation stage.
U.S. Pat. No. 4,124,688 (H. Shibazaki et al.) provides a process for preparing cubic calcium carbonate crystals of about 0.1 to about 1.0 .mu.m in size, which process comprises a first step of spraying a starting aqueous suspension containing calcium hydroxide and cubic calcium carbonate crystals into a carbon dioxide-containing gas, and a second step of adding calcium hydroxide to the aqueous suspension resulting from the first step and spraying the mixture into a carbon dioxide-containing gas.
U.S. Pat. No. 5,075,093 (H. Tanaka et al.) describes a two-step carbonation method of a milk of lime in which partially carbonated milk of lime is admixed with an aqueous carbonating solution containing an alkali metal carbonate or ammonium carbonate and an alkali metal hydroxide or ammonium hydroxide to complete the carbonation of calcium hydroxide. Particles having a platelet-like configuration of a thickness in the range of 0.1 to 0.3 .mu.m and dimensions within the plane of the platelet from 0.5 to 2 .mu.m are obtained.
Japanese Patent Disclosure (Kokai) No. 89-18911 relates to a process for preparing calcium carbonate spindle particles having an average minor axis of 0.1 to 1 .mu.m and an average major axis of 0.6 to 3 .mu.m in which a sulphate compound is added to a water slurry of calcium hydroxide and carbon dioxide is blown into the water slurry.